On the contrary, I suspect that our core beliefs, the ones we start with are the least rational. Logic and philosophical defenses come much later in the formation of a worldview. These arguments often only rationalize what we already think for personal, emotional reasons, based even on childhood experiences, private goals, social conditioning, and the like.

What discredits the monotheistic religions for me isn't so much their irrationality--although the creeds are of course irrational; more importantly, these religions fail on aesthetic grounds. They're obsolete so they no longer inspire the way a good story should. The adherents are left to defend them on political grounds.